Scanning Probe Microscopyfor Physics 9826a – Surface Science

Heng-Yong Nie (hnie@uwo.ca)

October 18, 20, 2010P&A 232

What is SPM?

Scanning Probe Microscopy is a family of the following microscopes:

Scanning Tunneling MicroscopyTunneling currentSurface morphology at atomic resolution in UHV (in principle)Local electronic structure on surfaceConductive required

Atomic force microscopy / Scanning Force MicroscopyAtomic force or tip-sample interaction Mechanical probe microscopyMeasuring almost any materials in any environment

AFM based techniques: probing mechanical, electrical, magnetic and thermal properties

Scanning probe microscopy (SPM) is a mechanical probe microscopy that measures surface morphology in real space with a resolution down to atomic resolution. SPM was originated from the scanning tunneling microscopy (STM) invented in 1981-83, in which electrical current caused by the tunneling of electron through the tip and the biased sample is used to maintain a separation between them. Because STM requires that the sample surface be conductive, atomic force microscopy (AFM) was developed in 1986 to measure surface morphology that are not a good conductor. AFM has since been developed very rapidly and has found much more applications than STM in many fields. The majority of the developments in nanotechnology will have to rely on SPMs in feasible future.

STM was invented in 1981

Heinrich Rohrer Gerd Binnig

1986 Nobel Prize in physics

AFM invented in 1986 by

Gerd BinnigCalvin QuateChristopher Gerber

An Omicron STM/AFM system

Scanning tunneling microscopy

constant current mode:current is the feedback parameter

By approaching the tip with a specified bias and current, the tip will be held at a certain distance from the sample surface so that the specified current (set point) is realized. By scanning the tip across the sample under this condition, the system compares the measured current I and the set point current Is (I-Is) and uses this error signal as the feedback parameter to apply an appropriate voltage to the z-piezo to adjust the tip-sample distance so as to diminish the error signal (i.e., I-Is 0), thus providing the height profile of the “topography” of the surface. This is the constant current mode. The other operation (constant height) mode is to keep the tip-sample distance while recording the current, which apparently requires the scanned area to be flat.

V

z piezo

FeedbackScanningData proc.

electronz profile

Amp LogAmp

Set point

Error

I

Sample

Tip

Piezoelectric scanners

Piezoelectric effect: electric field induced displacement of crystalline lattice and vice versa.

Material: lead zirconate titanate: PZT.

Powders are fired (1350º C) to form films. After polarization under an electric field (e.g., 60 kV/cm for an hour), they are used as scanner elements.

Calibration are necessary.

Curie temperature ~ 350ºOperated significantly below CT

www.physikinstrumente.com/

T>CT

T<CT

Unpoled During poling After poling

Vibration isolation is critical to achieve atomic resolution

Combination of spring suspension and eddy current damping brings an optimum damping for STM.

Probability of finding electrons on the other side of the barrier

Electron tunneling through a barrier

ψ12 2

2

0 2( )( )

em U E

w−−

h

Tunneling current scalesexponentially with the barrier width w

( ) ( ) ( )− + =h2 2

22m xU x E x∂

∂ψ ψWavelike behavior governed

by Schrödinger’s equation

Electrons incident to the barrier

ψ1 ( )x Ae Beikx ikx= + −

ψ ψ2 1 0( ) ( )x Ce De ekx kx kx= + =− −

ψ 3 ( )x Fe Geikx ikx= + −

k mE=

2h

k mE=

2h

κ =−2m U E( )

h

Electrons within the barrier

Electrons tunneling to the other side of the barrier

U

x0 w

E

Tunneling constant κ

Wave number k

Wave number k

Tunneling current between biased tip-sample

tipsample

Ef

wΦ

Evac

vacuum gap

Zero bias

tipsample

eV

─

I

Negative sample biasElectrons below Ef tunnel to tip

Electrons in the sample with energy within Esf-eV to Esf tunnel into the tip above its Etf to Etf+eV. This tunneling of electrons will be measured by the circuit connecting the tip and sample and used as the feedback parameter to maintain a constant current (setpoint).

Etf

Esf

VI e

m w∝

−ψ1

2 2 2

0( )Φ

h

Local density of states

By varying bias, the tunneling current becomes a measure of local density states for electrons

Tip far away

Tip close to sample (within tunneling distance)but no bias

Sample positively biased:Electrons from tip tunnel to empty states of sample

Sample negatively biased:Electrons from occupied states of sample tunnel to tip

P.G. Collins et.al., Phys. Rev. Lett. 82, 165 (1999)

Scanning tunneling spectroscopy

Schematic diagram of Orbital Mediated Tunneling Spectroscopy and a representative spectrum obtained from cobalt(II) tetraphenylporphyrin in an STM under UHV conditions at room temperature. The central diagram shows resonant tunneling through unoccupied (upper) and occupied (lower) orbitals in positive and negative bias, respectively. This diagram works equally well for a M-I-M junction (base and top metal labels) and for an STM (tip and substrate labels).

K. W. Hipps, in Handbook of Applied Solid State Spectroscopy, ed. D. R. Vij, Springer, Berlin, 2006.

Scanning tunneling spectroscopy

Tunneling current sensitive to tip-sample distance: feedback parameter

Distance (Å)

Cur

rent

(arb

. uni

t)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.5 1 1.5 2 2.5 3

~1.5 Å

Φ in eV and w in Å

With averaged work function ~4 eV,

I e w∝ −1 025. Φ

I e w∝ −2

When tip-sample distance changes by 1 Å, the tunneling current will change 7.3-10 times. A consequence of this sensitivity is the physics behind STM. Suppose an STM tip is terminated by a single atom (radius ~1.5 Å). The next atom is therefore ~2.6 Å away from the terminus atom, whose contribution is e-5.2=0.6% of the current contributed by the terminus atom. This high sensitivity of current over distance makes STM an imaging tool having atomic resolution (of course with good tips and samples).

90% of current fromthe protruded single atom

Tunneling resistance and current measured on Pt

G. Binnig et.al., App. Phys. Lett. 40 , 178-180 (1982)

I VR

em w

=−2 2 Φ

h

Phys. Rev. Lett. 90, 116101 (2004)

STM accepted as a tool being capable of probing atoms in real space

7 x 7 Si (111)1983

Being able to image Si (111) 7x7 STM breakthrough

Pys. Rev. B 70, 073312 (2004)

Phys. Rev. Lett. 50, 120-123 (1983)

Standard test sample for atomic resolution of STM and AFM

L. Lafferentz et al., Science 27 February 2009:

Vol. 323. no. 5918, pp. 1193 -1197

On-surface polymerization of DBTF to conjugated molecular chains. (A) Chemical structure of DBTF molecules with experimental (B) and calculated (C) STM images (2.5 by 4.5 nm) of intact molecules, with bright protrusions associated with the lateral dimethyl groups. (D) Overview STM image (80 by 120 nm, 1 V, and 0.1 nA) after on-surface polymerization. The produced long covalently bound molecular chains, i.e., polyfluorene, follow the herringbone reconstruction of the substrate. (E) STM image (5.9 by 3.6 nm) of a single polyfluorene chain end with its chemical structure superimposed (using a different scaling). The arrows indicate three identical (in the STM image and the chemical structure), newly formed covalent bonds between individual building blocks.

STM on polymerization of dibromoterfluorene on Au(111)

DBTF molecules (about 1/10 of a monolayer) were evaporated at a Knudsen cell temperature of about 500 K onto the Au(111) sample held at about 320 K.

When Pt atoms adsorb onto Ge(001) surface, they intermix with the substrate and locally lift the reconstruction, while submerging into the crystal. As a result of high temperature annealing of the Ge substrate at ~1000 K they partly re-appear at the surface, forming extremely well ordered nanowire arrays by self-organization. These wires are stable at room temperature.

Scanning Tunneling Spectroscopy / Microscopy, shows they are metallic, kinkless and defect free. They are only 0.4 nm thin with a spacing of 1.6 nm in-between and have aspect ratios up to 1000. Besides patches of nanowires, individual ones with varying lengths are also common on the surface.

Appl. Phys. Lett. 83, No. 22, 1 Dec. 2003.

Metal nanowire

To achieve atomic resolution, an STM probe has to be effectivelyterminated by a single atom

Tungsten tip: electrochemistry (anodic oxidation) in NaOH

A tungsten wire positively biased (relative to a circle of stainless steel wire) is thinned in NaOH by anodic oxidation and it eventually breaks by the weight of the lower part of the wire.

Annealing treatments are necessary to remove oxide left on the probe.

Still the overall probe size is tens of nanometers.

Scanning on surface, applying high voltage to the tip in the hope that a single atom protrudes over the tip

apex.

Keep scanning and perhaps purposely crash the tip to sample surface… Single atom terminating tip

NaOH

W

Anodic oxidation4-12 V

It is realized in the early days of STM that the tip impose forces on the sample when they are close to each other. This, plus the need to scan insulating samples, drove the effort in the invention of atomic force microscopy in 1985.

STM AFM

http://www.almaden.ibm.com/st/nanoscale_st/nano_science/STMAFM/design/

Shown here is the use of a tuning fork to measure both tunneling current and force. The tunneling current is measured as in STM and the force is measured by way of measuring resonance frequency change (details to come…). A very small oscillation amplitude must be used to avoid disturbing the tip-sample interaction (0.25 Å ).

Tunneling current & forces exerted on the probe: approach

http://www.almaden.ibm.com/st/nanoscale_st/nano_science/STMAFM/design/

Maximum attractive force ~2 nN

Tunneling current & forces exerted on the probe: results

STM operates in the attractive force region (tip-sample)

The cantilever is a metal foil and the tip is a piece of diamond glued on the foil. Naturally, an STM tip was used to detect the deflection of the cantilever. The deflection detection scheme was soon replaced by optical lever detection.

The first AFM

Atomic force microscopy: force used as the feed back parameter

AFM images surface features of a sample by scanning a sharp tip attached to a cantilever. The basic principle for AFM is schematically shown on the right. The key point in AFM is the measurement of the interaction between the tip and surface features, which is sensed through the deflection of the cantilever using a laser beam. There are two operation modes for AFM:

1) Contact mode AFM---the tip is mechanically contacted with sample surface at an applied force. Lateral force detection in this mode can be used to probe local surface properties.

2) Dynamic force (tapping, non-contact) mode AFM---It is the oscillation amplitude (or phase) that is used as the feedback parameter to keep the tip-sample interaction constant. Surface can be imaged without (or with less) surface degradation due to the elimination of lateral force in this mode.

Laser diodePhotodiode

Tip

Cantilever

Monitoring interactionbetween tip and surface

Keeping it constant by adjusting the separationbetween tip and surface

Height information

Attached to

scanner (x, y, z)

Contact mode

Dynamic force(Non-contact)(Tapping)

Contactforce

Oscillationamplitude

Sample

How AFM works...

Contact mode AFM: tip contacts with sample

Attractive forces: Capillary and van der Waals forcesRepulsive forces: Overlapping of electron clouds (Coulomb interaction)

T. Stifter et. al., Phys. Rev. B 62 (2000) 13667.

One can understand the interaction forces between the tip and the surface in AFM through the equation for Lennard-Jones potential w(r) = -A/r6 + B/r12, which deals with the interaction between two atoms. The force is thus F = -dw(r)/dr= -6A/r7 + 12B/r13. According to the text, A and B are known to be 10-77Jm6 and 10-

134Jm12, respectively. We show the calculated results here in two regions: 1) around 0.4 nm a small attractive force is seen and 2) when the separation between the two atoms gets close to 0.2 nm the repulsive force increases steeply. Note the difference in force scale in the two figures.

In practice, the AFM probe tip and the sample surface will have attractive force much larger than what is shown here. Also, other longer-range forces could occur. This is because the size of the tip is nominally ~10 nm nowadays and, in air, there are water films covering everything.

Interaction force between two atoms

What is behind AFM?

A tip-cantilever sensor systemThe interaction between the sharp tip and the sample surface is detected by the deflection (contact mode AFM) or the oscillating amplitude (dynamic force mode AFM) of the cantilever. A photodiode is used to detect the movement of the cantilever from a laser beam irradiated on the cantilever.

Two types of cantileverSoft cantilever (spring constant < 1 N/m) and stiff cantilever (spring constant > 5 N/m) are used in contact and dynamic force AFM, respectively. The geometry of cantilever is schematically shown (width for both types is 20-30 μm) between the two optical images for the two types of cantilever.

AFM cantilevers

~ 10 μm

100-200 μm0.6 ~ 4 μm

~ 10 nm

Optical detection scheme: Laser and position sensitive photodetector

Sensitivity: photodetector output deflection of cantilever (mV/nm)

Spring constant k Ewt=

3

34l

www.nanoscience.com/products/AFM_cantilevers.htmlF kz= −Force exerted

Cantilever detection of force exerted on tip

A soft cantilever (<1 N/m) is usually used. The tip is in contact with the sample surface. Imaging force can be selected at the repulsive force region at a couple of nanonewton (nN). Morphology is obtained through scanning the surface at a constant force.

Imaging mechanism of contact AFM

A force-distance curve shown here was obtained on a UV/ozone modified polymer surface with a cantilever having spring constant of 0.03 N/m. Interaction between the tip and the surface at different separation is indicated by a-f. Adhesion force between the tip and sample due to their contact can be measured as shown in the force-distance curve. Because the tip is in contact with the sample surface, damage on soft sample during scanning could happen. This can be largely avoided in the dynamic force mode AFM.

Force-distance curve

Dotted line indicates imaging force

Contact mode AFM

c

Adhesion force from force-distance curve

Adhesion force (nN)0 2 4 6 8 10 12 14 16

Cou

nts

0

4

8

12

16

20

b

Striped

"Normal"

Tip-sample separation (nm)0 50 100 150 200 250 300

Forc

e (n

N)

-8

-6

-4

-2

0

2 a

"Nor

mal

"

Stri

ped

MDMD

Topography

Friction force

norm

al

strip

ped

Force-distance curves are obtained by extending the tip to the surface to make a contact between the tip and the sample surface followed by retracting the tip from the surface. The original point for the distance may be defined as the mechanical contact between the tip and surface in the extending cycle. Extending the tip beyond that point will result in load forces applied to the surface. The slope of t his load force is a measure of the Young's modulus of the surface, possibly mixed with the spring constant of the cantilever. As a result, a cantilever whose spring constant is comparable with the surface stiffness should be used to measure the elasticity information.

In the retracting cycle, because of the adhesion properties between the tip and surface, the tip will not depart from the surface until the force used to pull the tip from the surface exceeds the adhesion force between them. This pull-off force can be considered as a measure of the adhesion force between the tip and surface. Adhesion force can be related to surface energies of the tip and sample surfaces, as well as their interfacial energy. Shown here is an example of measuring adhesion force at different regions on a polymer film. The striped areas have higher adhesion as well as friction forces than the normal surface.

20 µm

Friction force detection

During the scanning of the tip over an surface, the torsional movement of the cantilever can be recorded, which results in lateral (friction) force imaging. This imaging technique provides information on friction force or chemical force distribution on the surface. With some modifications on AFM, resistance distribution and mechanical properties can be obtained simultaneously with the morphology.

Ff=µFnFf

Fn

Scan direction

Scan direction

Phot

odet

ecto

rsig

nal

Torsional movement of cantilever: Friction microscopy

DistanceFric

tion

forc

e

The difference of the phtodetector signal corresponding to the two direction scans is a measure of friction force

Ff= ∆V×S×kt∆V: Torsional signal-difference (V)S: Cantilever torsional sensitivity (m/V)kt: Cantilever torsional spring constant (N/m)

Image with a contrast related to friction force

Quantitative measurement of friction force requires converting factors

Amphiphilicity of a molecule revealed by friction force imaging

Imaged by a Si tip, which is terminated by OH friction force on OPA headgroup is greater than on OPA tail.

Amphiphilicity of molecular layersExcessive amount of OPA molecules were observed to form bilayer and odd-numbered multilayers when their solution was placed on a Si substrate followed by a heat treatment at 50-60 EC.

Topographic (a) and friction force (b) images (scan area: 14.0 µm × 8.8 µm) showing bilayer and odd-numbered multilayers. Numbers shown in (a) indicate the number of molecular layer in the multilayers. Shown in (c) are profiles indicated by insert dotted lines in (a) and (b) for the height and friction force. The gray scale is 0 to 40 nm for (a) and 1.3 to 2.0 nA for (b), respectively. Distance (μm)

0 2 4 6 8 10 12 14H

eigh

t (nm

)0

5

10

15

20

25

Fric

tion

forc

e (a

rb. u

nit)

1.2

1.3

1.4

1.5

1.6

1.7

1.8

c)

9

7

5 2

753

a)

b)OPA molecules

32

5

n-Octadecylphosphonic acidCH3(CH2)17P(O)(OH)2

PHO OH

C

O

HeadgroupH

ydro

carb

on c

hain

D. Alsteens et al., Langmuir 23, 11977-11979 (2007).

If surface chemistry of the probe tip is known, then adhesion force mapping serves as chemical force microscopy

Chemical force microscopy: friction force microscopy with a functionalized tip

CH3

COOH

CH3-terminated tip

A. Noy et al., Ann. Rev. Mater. Sci. 27, 381-421 (1997).

Preferential oxidation on scratched areasLateral force imaging and adhesion force measurement are used to investigate preferential oxidation on scratched areas. It is clear from the friction force image shown below (scan area is 6 μm square) that the shear-force deformed PP areas have higher friction force (and higher adhesion force) than the unscratched area. The images (scan area is 1 μm square) on the right column show clearly preferential oxidation caused by ozone treatment on the scratched area.

Topography Friction force image

Unscratched Scratched

Orig

inal

Ozo

ne-tr

eate

d

Creating local active areas

Surface energy increase determined by AFM

Surface modification of polymer filmsAFM measurements clearly show formation of mounds on UV/ozone treated polypropylene (PP) film from the original surface characterized by fiber-like network structure (scan area is 2 μm square and height range is ~ 25 nm) and an increase in adhesion force. This increase in adhesion force indicates an increase in surface energy due to the oxidation of the modified polymer films.

Adhesion force increaseThe adhesion force distribution for original and UV/ozone treated PP films shown below was obtained from force-distance measurements. It is clear that the adhesion force was increased with the UV/ozone treatment time.

Modification of Surface morphologyand energetics on polymer surfaces

Original PP 15-min treated PP

Adhesion force (nN)2 4 6 8 10 12 14 16 18

Counts

0

5

10

15

20

25

30UV/ozone treatedPP filmsOriginal

3 min15 min

1 min

Adhesion force increase by surface oxidation

Formation of polar (functional) groupsby UV/ozone treatment

Unshared electron pair

Polypropylene (PP)

CH CH2

CH3

CH CH2

CH3

CH CH2

CH3

CH CH2

O

C CH2

CH3

CH C

CH3

OH O

OH

Carbonyl

Hydroxyl Carboxylic acid

Why adhesion force increases?

According to JKR theory, adhesion force F and work of adhesion w can berelated as F =-(3/2) B R w, where R is the radius of the tip.

Surface energy of the tip (1 =(1d +(1

p

surface energy of the sample surface (2 =(2d +(1

p

Superscription d and p denote the dispersion (non-polar) and polarcomponents of the surface energy, respectively.

Assuming that the geometric mean rule for the two components is true forthe work of adhesion as follows,

w = wd + wp = 2 ((1d (2

d)1/2+ 2 ((1p (2

p)1/2

we have the following relationship between the surface energy andadhesion force for an AFM system

F =-3 B R [ ((1d (2

d)1/2 + ((1p (2

p)1/2]

Assuming the surface energy of the tip remains unchanged, then an increaseof adhesion force indicates an increase of surface energy of the sample.

Relating adhesion force (F) to surface energy (( )through the work of adhesion

Young’s modulus estimated from force-distance curves

By modulating distance and recording response of the cantilever under contact mode, modulus contrast mapping is possible.

[ ]

h p dF k pF h k h d h

c

c

= +=

= −( ) ( )

d h F h D R( ) ( ) ( / )=23 2

13

DE Es t

= − +⎛⎝⎜

⎞⎠⎟

34

1 1 12( )σ

F h k h F h D Rc( ) ( ) ( / )= −⎡

⎣⎢

⎤

⎦⎥

23 2

13

h F hk

F h D Rc

= +( ) ( ) ( / )

23 2

13

With h being the piezo movement, p the deflection of the cantilever and d the penetration of the tip to sample. The cantilever’s spring constant is kc and the force F exerted on the tip can be calculated from the cantilever deflection

Hertzian model connects penetration, force, the radius of the probe, Young’s moduli of probe and sample, as well as Poisson’s ratio.

The relationship between the force and piezo movement is thus:

Photo diode position sensor for detection of cantilever oscillation via a lock-in amplifier, which is a measure of Young’s modulus

Cantilever oscillation over area 2Cantilever oscillation over area 1

Bimorph

~Power source forsample oscillation time

ampl

itude

time

ampl

itude

time

ampl

itude

Sample stage oscillation

1 2

Force Modulation technique to image elasticity distribution

In contact mode AFM, together with the topographic feature, one can probe local elastic properties of materials through a mechanical interaction between the surface and tip. This can be done by oscillating the sample height while measuring the response of the cantilever with lock-in amplifier technique. In this technique, what is measured is the slope of a force-distance curve, which corresponds to elastic properties of the sample. Therefore elasticity difference on a surface can be distinguished by using this technique. Shown in figures to the left are topography (left) and elasticity (right) mapping for polystyrene (above) on a mica substrate and a polystyrene/polyethylene oxide blend film. Scan area is 5.25 and 3.75 μm square for the polystyrene and polystyrene/polyethylene oxide blend samples, respectively.

Force Modulation

Force-distance curves (a) obtained on the mica and the PS film and the simultaneously obtained response (b) of the cantilever to an oscillation of the sample height with an amplitude of 1 nm at 5 kHz. The spring constant of the cantilever used was 18 N/m and the approaching and retracting speed for the tip was 3 nm/s. The difference seen for the cantilever response (b) is due to the different slope of the force-distance (a) curves on the different materials, which is a reflection of difference in Young’s modulus for mica (200 GPa) and PS (5 GPa).

N. Almqvist et al., Biophys. J. 86, 1753–1762 (2004).

Force volume mapping: low pixel density (64 x 64) because a force-distance curve is taken at each pixel.

Force volume mapping: Young’s modulus and adhesion force

Morphology Young’s modulus

Dynamic force mode AFM

Cantilever is vibrated at a frequency close to its resonance.

The amplitude of the vibration will be reduced when the tip is in the regime where tip-sample interaction exists. A reduced amplitude is used as the feed back parameter to follow the morphology of the samples surface.

Moreover, surface properties such as viscoelasticity and surface chemistry can be visualized simultaneously with the morphology

tmFz

dtdz

Qdtzd ωωω cos02

00

2

2

=++mk /2

0 =ω

)cos( φω += tAz

Point mass – spring model for AFM: Equation of motion for forced damped oscillation in free space

tFkzdtdzc

dtzdm ωcos02

2

=++mass

damping coefficient spring constantdriving force

Qmc /0ω=

amplitude phase shift

quality factor

Steady state solution

Q mkc

=

2/120

2220

0

])/()[(/

QmFAωωωω +−

=

Frequency (kHz)260 280 300 320 340

Am

plitu

de (a

rb. u

nits

)

0

1

2

3

4

5

6Q=500ω0=300 kHzk=40 N/m

tan( )/

φωωω ω

=−0

02 2

Q

Frequency (kHz)260 280 300 320 340

Phas

e (d

egre

e)

0

45

90

135

180

Resonance frequency and phase lag (frequency sweep)

Amplitude Phase

Oscillating cantilever to detect force gradient between tip and sample

Resonance frequency changes when there is force exerted to the tip:

Causing changes in amplitude and phase lag.

Thus, amplitude-modulation AFM and phase imaging.

fk F

zm

=+

2π

∂∂

J. Appl. Phys. 69 (2), 15 January 1991

f km0 2= π

Δfk F

zm

km

f Fk z

f Fk z

f Fk z

=+

− = + −⎛

⎝⎜

⎞

⎠⎟ ≈ + −⎛

⎝⎜⎞⎠⎟=2 2 1 1 1

21

20 0 0π

∂∂ π ∂

∂∂∂

∂∂

Imaging mechanism of dynamic force AFM

Amplitude-distance curve

Dotted line indicates imaging condition

(amplitude is damped to ~ 50 %)

A stiff cantilever (~40 N/m) is oscillated around its resonant frequency (see the insert in the figure shown here). The tip is not in contact with (very small amplitude, say 1 nm; this is non-contact mode) or only taps (large amplitude, usually > 10 nm; this is tapping mode) the sample surface. That way, lateral dragging due to scanning is largely eliminated, effectively preventing soft sample surface from being damaged as could be using contact mode AFM. Information available from this mode: Morphology, phase imaging (due to differences in friction force, chemical force, adhesion force, mechanical properties).

An amplitude-distance curve should look like the one shown on the right. Interactions between the tip and the surface at different separation are indicated. In free space, the tip is far away from the sample. After the tip is in contact with the sample, the cantilever no longer oscillates. In the working area, the amplitude is reduced as a function of tip-sample separation, which is used as the feedback parameter to obtain surface morphology. Usually, surface is scanned by maintaining a reduced amplitude at 50 %.

Tip-sample distance zt

A/A

0

Working area

1

0

Free space

Sample

z t

A0

Contact

A

A0Cantilever is vibrated by applying a sine wave at a frequency f (close to resonance frequency) to a piezo device on which the cantilever is attached. In free space, the cantilever oscillation amplitude is A0 (nm).

AFM image of a grating (26 nm height standard) Topview

3-D Profile

3.0 μm

26 n

m

Examples of AFM image

1 µm × 1.5 µm

Example of AFM image: metal oxide

High spatial resolution

Phase imaging

In dynamic force mode AFM, phase shift between the sinusoidal voltage source applied to oscillate the cantilever and the actual oscillation of the cantilever can be effected by the interaction between the tip and the sample. The phase shift in the oscillating cantilever is related to tip-surface interaction which is basically material specific. Therefore, phase shift in tapping AFM can be used to distinguish different surface compositions on a surface (see the schematic below). There are many surface properties that may have an effect on the phase shift contrast. They could be differences in friction, viscoelasticity, adhesion, material, etc. Phase imaging usually gives clear contrast on a surface if there are any differences in surface properties as described above. Applications include visualizing phase separation in polymer blends, distinguishing different compositions on surface. Shown here is topography (left) and phase image (right) for a surface of a toner particle of carbon black matrix with polymer filler (scan area is 3.5 μm square).

Phase difference between area 1 and 2

1 12 2

Applied voltageResponseOn area 1

On area 2

Phase shift curves obtained with a cantilever of k = 2 N/m and Q = 150 on a silicon substrate

N.F. Martinez and R. Garcia, Nanotechnology 17 (2006) S167–S172

Phase shift is sensitive to tip-sample interaction. This is why phase shift image is a powerful technique in differentiating component materials on a sample.

According to Cleveland et al. and Garcia et al., phase shift angle φ is related to tip-sample energy dissipation (Edis) process (which in turn reveals difference in material properties) and hydrodynamic damping in the medium (Emed). In amplitude-modulated AFM with a cantilever oscillated at frequency ω (ω0 being resonance frequency) having a free amplitude A0 and setpoint A, the phase lag φ is

⎟⎟⎠

⎞⎜⎜⎝

⎛+=

med

dis

EE

AA 1sin

00ωωφ

Phase shift angle dissipation energy at tip-sample interaction

Phase shift image and energy dissipation

Molecular layers (Sexithienyl, T6) deposited on silicon

Dissipation energy at different tip-sample separation normalized to free amplitude.

E AA

AA

kAQdis = −

⎛⎝⎜

⎞⎠⎟sinφ π

0 0

02

N.F. Martinez and R. Garcia, Nanotechnology 17 (2006) S167–S172

SiO2

HS(CH2)11(C2H4O)3OH

HS(CH2)11NHCOC9H15N2OS

Glycol

Biotin

Topography Phase image

Phase imaging sensitive to surface chemistry

12 μm

Two thiols terminated by different functional groups are self-assembled on Au films

Topography Phase image

AFM phase imaging is a powerful technique to visualize additives in a polymer matrix due to their significantly different viscoelasticity. Shown here is an example of comparing two samples made under different conditions. Phase images in the right column display striking contrasts between the additive and the matrix. It is immediately clear that the dispersion of theadditive is quite different in the two samples.

Phase imaging: revealing fillers in polymer matrix

Polymer matrix

Fillers

2 μm

Phase imaging and imaging ToF-SIMS of rat neurons

ToF-SIMS analysis of neuronsof a sectioned rat brain film using an IONTOF instrument

Animal samples were provided by the Autism Research Group at UWO led by D.F. MacFabe. The results shown in this slide were from a collaboration between Surface Science Western and the ARG.

SIMS is a method of analyzing the mass/charge ratio of ionized particlesproduced from the sample upon bombardment of an energetic primary ion beam.

CH3PO4¯

C2H4PO4¯

NC2H4PO4¯

(CH3)3NC2H3PO4¯

Mass (m/z)

Inte

nsity

(cou

nts)

CH4NC2H4PO4¯

110 120 130 140 150 160 170 180 190

5x10

0.20.40.60.81.0

CN¯ PO2¯CH¯

a b c

fed

5 µm CH¯+CN¯+PO2¯

Topography Phase shift ToF-SIMS

AFM topographic (a) and phase shift (b) images (22 μm × 22 μm) for neurons in the CA1 field obtained on a sectioned rat brain film (thickness: 30 μm). The time-of-flight secondary ion mass spectrometry (ToF-SIMS) negative secondary ion images of (d) CH¯, (e) CN¯and (f) PO2¯ are overlapped in (c) where CH¯ is plotted in blue, CN¯ in green and PO2¯ in red, respectively. The ion images were measured using Bi+ primary ion beam with the high spatial resolution (burst alignment) mode. The ion images are plotted using 600 scans of data collection. The white lines inserted in (a)-(c) guide the eye to subcellular features imaged by both techniques.

Accuracy of AFM measurement: Convolution

Convolution of tip geometry into measured features when tip apex radius is comparable with dimension of surface features.

05

1015202530354045505560

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

R=10 nm R=5 nm R=20 nm

2r (nm)

w (n

m)

r

R

r

R

x Rc

w

( ) ( )x R r R r Rr2 2 2 4= + − − =

( )R Rr R rc c2 24 2= + −

R R rc = +

w Rh= 2 2h r= 2

R=r

r x x rc = + = +1 21(

costan )

θθθr

rx1x2

x r1 = / cosθx r2 = tanθ

rc

1.05

1.10

1.15

1.20

1.25

1.30

1.35

5 6 7 8 9 10 11 12 13 14 15

θ, degree

r c/r

When tip radius « dimension of surface feature, the overall tip shape (cone angle), rather than the tip apex, dominates the convolution.

Accuracy of AFM measurement: Convolution

The morphology of a surface imaged by AFM is obtained through an interaction between the probe tip and surface features. When the tip is contaminated and the size of the contaminant is comparable to or larger than the size of the features on the sample surface, artefacts attributable to the contaminant are observed to dominate the image (see the following figure on the left-hand side: dotted lines represent the measured profiles). Here is a simple and effective method of evaluating tip performance by the imaging of a commercially available biaxially-oriented polypropylene (BOPP) film, which contains nanometer-scale sized fibers. The BOPP film surface is appropriate for use as a reference because a contaminated tip will not detect the fiber-like network structure. The very fine fiber-like structure of the BOPP film surface is a good criterion for the tip performance as shown in the following figure (right-hand side).

Because the polymer film is soft compared to the silicon tip (Young's modulus for polypropylene is 1-2 GPa, while for silicon it is 132-190 GPa), the polymer will not damage the tip when the tip is pushed into the polymer. This property can be used to clean a contaminated tip, i.e., by pushing the contaminated tip into the polymer, contaminants are most likely removed from the tip apex, probably to the side of the tip. Another important property of the BOPP is that the polymer film is highly hydrophobic and has a very low surface energy of ~ 30 mJ/m2 (The surface energy for Si is ~ 1400 mJ/m2; and the surface tension of water is 72 mJ/m2). These properties prevent contaminants from accumulating on the surface and hence prevent the contamination of the tip in the evaluation process.

Accuracy of AFM measurement

The smaller object images the larger one: thus tip effect possible

An AFM image (a) obtained on a BOPP film using a clean tip, reflecting the true morphology characterized by the fiber-like network structure. When damaged or contaminated AFM tips were used, the fiber-like features are no longer seen in the AFM images [ (b)- (d)]. These three images are obviously dominated by three different tip shapes. Comparison of the AFM images strongly suggests that the BOPP film can be used as a reference sample to check the performance of an AFM tip.

The criterion is simple and straight forward: if the fiber-like features are revealed by an AFM tip, then the tip quality is sufficient to collect “true” images.An AFM tip can be easily contaminated or damaged depending on the chemical and mechanical properties of the sample surface it scans. It is therefore desirable to adopt a simple qualifying method such as this one using BOPP film to check the performance of the AFM tips to make sure the collected AFM images be meaningful.

BOPP as a test sample to check AFM tip performance

Only this image is “true”

a

b

c

In order to prove that BOPP film is indeed useful for checking AFM tip performance, we managed to image the same area using the same tip when it was clean, after it had been contaminated and then after it was cleaned again. That way, any change in the AFM images obtained would be solely due to the contamination on the tip apex.

AFM images shown in (a), (b) and (c) were obtained on the same area of a BOPP film using the same tip when it was clean, contaminated and re-cleaned, respectively. The tip was contaminated by being scanned on an organic acid coated Si substrate. It is clear that the image collected by the contaminated tip is dominated by tip effect (b). We cleaned the contaminated tip by pushing it into the polymer film, and its cleanliness is evidenced in AFM image shown in (c). Therefore, the use of the BOPP film to check the AFM tip performance and to clean the contaminated tip was successful.

Cleaning contaminated tip on BOPP

How to clean a contaminated tip

The amplitude versus the tip-sample separation when the tip is brought to and retracted from the sample surface is represented by open and filled circles, respectively. For clarity only the first and fourth curves are shown here. The speed of the tip movement was 100 nm/s.

Use of the morphology of BOPP as a standard to check probe performance (and even to estimate the tip geometry).

Tip effect is unlikely to be completely removed from the image.

Use of BOPP film: Application to blind tip reconstruction

If a material contaminating the tip apex is sticky (active in producing extra forces), the contaminated tip may causes noises in AFM imaging.

AFM tip contamination

-6 -3 0 3 6 9

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Tip-sample distance (nm)

A/A

0

contaminated tip

cleaned tip

AFM imagesAmplitude-distance curve

Contaminated tip

Techniques based on AFM

Scanning surface potential microscopy

Piezoelectric force microscopy

Magnetic force microscopy

Scanning thermal microscopy

Multimodal AFM – make use of dynamics and harmonics of oscillating cantilever

Frequency-modulation AFM (in UHV for atomic resolution)

Scanning surface potential microscopy

When there is a difference in potentials between the tip and sample surfaces, an oscillating electromagnetic force appears between the tip and sample surface at the frequency of the applied sinusoidal voltage, which makes the cantilever oscillate. This oscillation is used as the feedback parameter for the system which tries to stop this oscillation by applying a dc voltage to the tip (Vt) so as to make the potential difference between the tip and sample surfaces vanish (Vt+Vs=0). Thus the potential on the sample Vs can be measured.

Lift mode: scanning a line under tapping mode AFM to obtain profile, followed by repeating the same line scan with the tip lifted (e.g., 50 nm) with the AFM feedback being turned off and the SSPM feedback turned on.

Laser diodePhotodiode

Tip

Cantilever

Attached to

scanner (x, y, z)

Sample

~Vacsin(ωt)

Lock-inamp. (ω)

CantileverOscillation ? Vt

Grounded Au

Biased AuGlass

Bias 5.7 -5.7 V

F V V V tt s ac∝ +( ) sinω

An example of SSPM Application

http://spie.org/x8599.xml?ArticleID=x8599

AFM image shows the device topology and SSPM image reveals significantly more information about the potential distribution and current flow through the nanowire. Specifically, the SSPM image shows that the electroactive defect created by the electron beam also includes the area of the neighboring oxide, which is evident as the white area in the center. This image also shows that the defect significantly modulates the potential distribution along its length of the nanowire, indicated by the sharp white-to-dark transition along its length. In fact, most of the potential drop is in the area of the defect and is due to a p-n junction, which the electron beam created in the nanowire. This result illustrates that the defect-creating technique can be used both to study and control the function of nanodevices.

Contact mode AFMApplying ac voltage to the tip

Piezoeletric domain respond to the applied voltage by extracting or expanding; this mechanical response exerts forces to the contacted tip, resulting in detection of the deflection and torsion of the cantilever.

Amplitude for strength and phase for polarization direction.

This technique is used to detect ferroelectric domains as well as write on ferroelectric film.

http://www.ntmdt.com/spm-principles/view/piezoresponse-force-microscopyanimation

Laser diodePhotodiode

Tip

Cantilever

Attached to

scanner (x, y, z)

Sample ~V0sin(ωt)

Lock-inamp. (ω)

Def. Tor. Amplitude

Phase

Amplitude

Phase

In planeferroelectric

Out-of planeferroelectric

P

V0sin(ωt)

Piezoresponse force microscopy

Lateral PFM (a-amplitude, b-phase) and vertical PFM (c-amplitude, d-phase) images of a-c-domain structure in PbTiO3 film.

http://physics.unl.edu/~agruverman/gallery/image6.shtml

An example of PFM application

Scanning thermal microscopy

Probe thermal conductivity over the scanned area with a tip capable of measuring temperature. By elevating the tip temperature slightly higher than the sample, the tip temperature will drop during scanning (in contact with the sample) according to the thermal conductivity of material component.

Thermal image of a commercial carbon fibre

Journal of MicroscopyVolume 205, Issue 1, Pages 21-32 (2002)

Magnetic force microscopy

Use of a magnetized probe to detect magnetic domains on the sample by detecting frequency/phase shift of the oscillating cantilever caused by magnetic force gradient from sample.

Bits on a magneto-optical disk. The left image shows surface topography with tracks delineated by grooves. The magnetic force gradient map (right) was taken with LiftMode. Bit edge roughness is clearly visible, as is virgin domain structure in the grooves with features as small as 50nm. 5µm scan.

www.vwwco.com

Δf f Fk z

= 0 2∂∂

http://www.nanosensors.com/Magnetic_Force_Microscopy.pdf

MFM on hard disk: revealing recorded bits

Topography Frequency shift

Time-varying forces reconstructed from higher harmonics measurement

Torsional harmonic cantilever

Vertical vibration for feedback parameter to follow topography and torsional vibration for coefficients of Fourier transform (in frequency domain) to calculate back to time domain: time-resolved tip-sample interaction force.

O. Sahin et. al., Nature Nanotechnology 2, 507 - 514 (2007)

O. Sahin et. al., Nature Nanotechnology 2, 507 - 514 (2007).

Harmonics are calculated using FFT for F-D reconstruction

Force-time

Force-distance

Measuredtorsional signal

Discrete FT

Reconstructed

One-cycle of oscillation

f

z

Dissipation = dtfdtzdf

T rr•∫

0

Stiffness: DMT model

Material properties can be imaged oncea force curve is reconstructed at a pixel

Parameters extracted from the reconstructed force curves:High pixel density, real time with conventional imaging

Young’s modulus Adhesion force

Conventional force volume approach is slow and low pixel density

O. Sahin and N. Erina, Nanotechnology 19, 445717 (2008).

Reconstructed indentation force curves

Deduced Young’s moduli

O. Sahin et. al., Nature Nanotechnology 2, 507 - 514 (2007)

Frequency-modulation AFM

Block diagram of the frequency-modulation AFM feedback loop for constant amplitude control and frequency-shift measurement.

Rev. Modern Phys. 75, 949 (2003)

Δf f Fk z

= 0 2∂∂

Resonance frequency change is used as the feedback parameter to measure the surface.

H. Holscher et.al., Phys.Rev. B 61, 12678 (2000)

Si(111) 7x7

First non-contact AFM revealing Si(111) 7x7

F. J. Giessibl, Science 267, 68 (1995) www.afm.eei.eng.osaka-u.ac.jp/en/research/lecture/pic/7x7.jpg

Atomic resolution routinely obtained

FM-AFM: Atomic resolution

Chemical identification of individual surface atoms by FM-AFM

Frequency-modulation AFM in UHV allows precise measurement of attractive force.

Y. Sugimoto et.al., Nature 446, 64-67 (2007)

Schematic illustration of AFM operation in dynamic mode (a), and of the onset of the chemical bonding between the outermost tip atom and a surface atom (highlighted by the green stick) that gives rise to the atomic contrast (b). However, the tip experiences not only the short-range force associated with this chemical interaction, but also long-range force contributions that arise from van der Waals and electrostatic interactions between tip and surface (though the effect of the latter is usually minimized through appropriate choice of the experimental set-up). c, Curves obtained with analytical expressions for the van der Waals force, the short-range chemical interaction force, and the total force to illustrate their dependence on the absolute tip–surface distance. d–e, Dynamic force microscopy topographic images of a single-atomic layer of Sn (d) and Pb (e) grown, respectively, over a Si(111) substrate. At these surfaces, a small concentration of substitutional Si defects, characterized by a diminished topographic contrast, is usually found. The green arrows indicate atomic positions where force spectroscopic measurements were performed. Image dimensions are (4.3 4.3) nm2.

Nature 446, 64-67 (1 March 2007)

Difference in attractive forces leads to identification of atoms

SPM

STMAtomic resolution

Local density of states for electronEmpty or filled electronic state imaging

AFMAtomic resolution ultimately

Nanometer resolution (atomic resolution ultimately)Probe various surface properties

Platform for Nanotechnology